Aqueous coating binders based on epoxy resins have been used in the coatings industry for many years. Two-pack epoxy coating compositions, both solvent-borne and water-borne, are used particularly for heavy duty corrosion protection of metal substrates, with preference on base metals that do not have other corrosion protection such as zinc coating. Such systems dry quickly providing a tough protective coating with excellent hardness. Coating compositions based on epoxy resins are used mainly for factory-cast iron, cast steel, and cast aluminium parts. The use of water-borne epoxy resin systems reduces exposure and flammability issues associated with solvent-borne coatings, as well as liberation of solvents upon application. Coating compositions based on epoxy resins are mainly used in automotive and industrial applications, such as pipelines, and fittings, due to their heat resistance, chemical resistance and also, mechanical strength. Other uses of epoxy resin-based coating compositions are, for example, in can coating for acidic goods. Coating compositions based on epoxy resins are also widely used as primers to improve the adhesion of paints especially on metal surfaces in automotive and marine applications where corrosion resistance is important. They can also be used for high performance and decorative flooring applications such as industrial floorings, and architectural floorings such as terrazzo.
Water-based epoxy coating compositions usually comprise a hydrophilically modified epoxy resin, and a compatible curing agent which itself is also hydrophilically modified. Hydrophilic modification of epoxy resins is usually effected by introduction of non-ionic hydrophilic moieties. The reason is that the commonly used ionic hydrophilising groups such as amino groups or acid groups which form ions in aqueous environment are reactive themselves with epoxide groups. Introduction of the commonly used poly(oxyethylene) blocks as hydrophilising moiety is a difficult step as strong acid catalysts, mostly Lewis acids such as boron trifluoride, or complexes thereof with ethers or amines, have to be used, and the process is difficult to control. Such chemistry has been described in EP 0 272 595 B1, and also, in EP 0 346 742 B1, for epoxy resins. Hydrophilic modification of curing agents based on adducts of epoxide-functional compounds and amines has been described, i.a., in EP 0 000 605 B1. Amine based curing agents for epoxy resins usually have primary, secondary, or also tertiary, amino groups that react with an epoxide group under formation of a beta-hydroxy amine structure or a betaine structure. The curing activity decreases from primary to secondary to tertiary amines. While it is possible to use multifunctional primary amines which are the most efficient amines due to their higher reaction rate, such as isophorone diamine or meta-xylylene diamine, as curing agents for epoxide-functional compounds, their high vapour pressure and unfavourable smell together with potential health hazards has barred their use in applications where no sufficient ventilation is available. Moreover, lack of compatibility of monomeric amines with epoxy resins has limited their usefulness. Secondary amines which stem from reaction of primary amines with epoxide-functional compounds have good compatibility with epoxy resins, yet suffer from lower curing speed compared to that of primary amines.
Polyamines containing ketimine blocked primary amine groups which free primary amine groups upon hydrolysis have been described in GB 2028830A, describing the use of those amines for the Michael addition reaction under curing conditions with compounds comprising alpha,beta ethylenically unsaturated groups in conjugation with carbonyl groups. The resinous compositions described therein can be used neat or in organic solvent or can be dispersed in water by converting the amine-groups to cationic groups with the aid of acetic acid.
However, the dispersions of polyamine resins described in GB 2028830A have the disadvantage of having a very low solid content. Moreover, when used as hardener in association with epoxy resins, the polyamine dispersions disclosed in GB 2028830A do not permit to obtain coatings having satisfactory water-resistance. Hence, the polyamines described in GB 2028830A are not suitable to be used as curing agent for epoxy resins in aqueous dispersions.
Hydrophilically modified multifunctional primary amines which are derived from adducts of epoxide-functional compounds and amines have been described in co-pending PCT application PCT/EP2012/058172. By a “hydrophilically modified” compound in the context of this PCT application, a chemical compound (including oligomeric or polymeric substances having a number average molar mass of at least 350 g/mol) is meant which comprises in its molecules, moieties of oligo- or polyoxyethylene segments, optionally in mixture with polyoxypropylene segments, in a sufficient amount to keep the said compound stably dispersed in aqueous dispersion for at least one week at room temperature (23° C.), i. e. without phase separation or formation of precipitate visible with the naked eye.
The synthesis of the preferred hydrophilically modified multifunctional primary amines described in this patent application involves a pre-reaction where a hydrophilic epoxy-functional intermediate is made by reacting a stoichiometric excess of a multifunctional epoxide such as the diglycidylether of bisphenol A, with a water-compatible di-hydroxy functional aliphatic polyether such as polyethylene glycol (alpha,omega-dihydroxy polyethyleneoxy ethylene, CAS No. 25322-68-3), its propylene analogue (alpha,omega-dihydroxy poly-1,2-propyleneoxy 1,2-propylene, CAS No. 25322-69-4), and mixtures or copolymers thereof, under catalysis with strong acids such as boron trifluoride or complexes thereof. This intermediate is then chain-extended by reaction with a multihydric phenol, preferably also bisphenol A, and the resulting multifunctional epoxide product is reacted with an amino compound which has at least two primary and at least one secondary amino group, where all primary amino groups are blocked by pre-reaction with a ketone to form a ketimine, leaving only the secondary amino groups as reactive amino groups.
It has been found in the experiments on which the present invention is based that due to the incorporation of the hydrophilic segment into the multifunctional amine, the variability of the system comprising an epoxy resin and the hydrophilically modified aminic curing agent which serves both as curing agent and as emulsifier for the epoxide resin, has become unduly limited.
Moreover, the aqueous dispersions of the multifunctional amines described in this patent application also suffer from the drawback that only low solid concentrations can be obtained. In addition the water resistance of epoxy coatings based on such hardeners should be improved.
It has therefore been the object of this invention to provide an aqueous dispersion of at least one multifunctional primary amine as curing agent suitable for curing epoxy resins which overcomes those problems.
It has further been an object of this invention to provide an aqueous dispersion of at least one multifunctional primary amine for epoxy resins, and an emulsifier which is able to emulsify the curing agent and, optionally, the epoxy resin, thus dispensing the need for internal hydrophilisation of either or both of the epoxy resin. In such system, the emulsifier has to exhibit good compatibility, and thus, emulsifying power for both the epoxide functional compound which constitutes the epoxy resin, and the amine-based curing agent therefor.
The problem has been solved by providing an aqueous dispersion comprising at least one non-ionic emulsifier and at least one multifunctional amine having at least two primary amino groups, and moieties derived from epoxy resins. These dispersions of multifunctional amines can be combined as curing agents with epoxy resins, and be dispersed in water, optionally in the presence of a further emulsifier.